1. Field of the Invention
The present invention relates to an arc welding apparatus, a constant voltage characteristic welding power source, and a method for performing arc welding.
2. Description of the Related Art
A type of arc welding specialized in vertical-up welding in which high efficiency is obtained by filling a surface of a welding groove through a one-pass operation is called “electrogas arc welding” including an apparatus, and has been widely put into practice (for example, refer to Japanese Unexamined Patent Application Publication No. 59-130689, Japanese Patent No. 3596723, and Japanese Unexamined Patent Application Publication No. 2004-167600).
In the basic mechanism of the apparatus, a welding torch, a wire feed motor, a sliding copper plate including a gas supply port and a cooling tube, and a dedicated device (welder) including a lifting motor are mounted on a rail extending on a steel plate in a longitudinal direction of the groove, and welding is performed in the groove while lifting the welder in accordance with the rise of a surface of a molten pool.
For such welding, some methods for linking the rise of the surface of the molten pool with the rise of the dedicated device have been proposed.
In a first method, a link mechanism is not used. That is, this is a completely manual method in which the speed of the lifting motor is manually adjusted while constantly observing the surface of the molten pool. The configuration of the apparatus is the simplest, but constant observation is required. The workload is too heavy and the quality of welding obtained is too unstable to perform welding of a welding length of several meters to tens of meters.
Therefore, in order to reduce the workload and stabilize the quality of welding, an automatic rising mechanism has been proposed.
That is, a second method is a control method in which an optical sensor is mounted on the apparatus and a linkage with the rising speed of the surface of the molten pool is established by utilizing a mechanism in which the intensity of arc light changes in accordance with the rise of the surface of the molten pool.
However, this method is not stable since the arc is not always stable and affects the intensity of light and fumes (smoke) generated during the welding blocks the light irregularly.
Therefore, as a third method, a method in which feedback to the rising speed is performed using values of current may be used. This is currently the most popular method.
In this method, a constant voltage characteristic welding power source is used as a welding power source. When a wire feed rate has been determined, the constant voltage characteristic welding power source outputs a current large enough to melt a wire. The welding wire is melted by total energy of arc heat proportional to the product of the current and the potential difference of the arc (arc voltage), the electrical resistivity of the welding wire itself, a welding distance, and the product of the square of the current, and, as a result, the current and the welding distance are balanced. Here, because the welding distance becomes shorter and resistance heat decreases when the surface of the molten pool rises, the welding power source increases the output current thereof in order to supplement insufficient melting energy. Therefore, by instantly reading this increase in the output voltage and increasing the rising speed of the welder, the welding distance becomes longer again and the resistance heat increases. Because the melting energy becomes excessive, the output current is reduced, thereby suppressing the rising speed. By repeating this procedure continuously, the rise of the surface of the molten pool and the rise of the welder are linked to each other, and accordingly monitoring is no longer required.
That is, currently, only the feedback control of the current and the rising speed has been put into practice, and an arc length, which is an essential factor in the quality of welding, is not controlled at all except for the automatic control characteristic of the constant voltage power source. The automatic control characteristic of the constant voltage power source may be referred to as a function of maintaining a set arc length regardless of whether or not the set arc length is appropriate.
Voltage as a parameter in welding may be regarded as equivalent to the arc length, but in vertical-up welding of a large welding length, the arc length and the voltage need to be considered more strictly than in downhand welding or horizontal welding. This is because of differences in an arc force direction and a penetration direction.
FIGS. 11A and 11B are schematic diagrams illustrating a welding direction, the arc force direction, and the penetration direction in each welding attitude. FIG. 11A illustrates downhand welding, and FIG. 11B illustrates vertical-up welding. Hollow arrows indicate the welding directions, broken-line arrows indicate the arc force directions, and thick-solid-line arrows indicate the penetration directions.
As illustrated in FIG. 11A, in downhand welding, the arc force direction and the penetration direction in the groove are parallel to each other. Therefore, deep penetration is structurally easy to obtain, and few failures occur in penetration. Although it is known that the concentration of an arc depends on the arc length and accordingly the arc length affects the penetration, the degree of effect is small compared to that in vertical-up welding, which will be described hereinafter.
On the other hand, as illustrated in FIG. 11B, in vertical-up welding, the arc force direction and the penetration direction in a width direction of the groove are perpendicular to each other. That is, the arc force does not directly affect the penetration. In vertical-up welding, penetration in the width direction is obtained due to convection generated in the molten pool immediately below the arc. Therefore, the intensity of the convection determines the depth of penetration, and the penetration is shallow relative to input heat energy. Moreover, because the convection in the molten pool is easily affected by the distribution of arc force, small variations in the arc length cause failures in the penetration. That is, in vertical-up welding, appropriate control of the arc length is significantly important in terms of securing the quality of welding.
In the downhand welding illustrated in FIG. 11A, the penetration is generally deep when the arc length is small, whereas in the vertical welding illustrated in FIG. 11B, the penetration is deep when the arc length is large. These opposite characteristics derive from the above-described difference between their respective mechanisms.
Currently, however, the arc length and the voltage are not controlled at all as described above. Since the arc length is generally considered equivalent to the voltage, a value of voltage corresponding to a certain value of current is managed as a model condition, but a problem arising in this case is the reliability of the absolute value of voltage.
FIGS. 12A and 12B are diagrams illustrating relationships between arc voltage and a voltage loss in cables.
As illustrated in FIGS. 12A and 12B, power source output voltage Vpower output from a welding power source includes not only arc voltage Varc, which is the potential difference of an arc, but also a voltage loss ΣVcable in secondary cables connecting the welding power source and a welding torch and the welding power source and a base metal or connecting portions (Vpower=Varc+ΣVcable).
That is, in the secondary cables and the connecting portions, part of power is converted into a heat loss due to the voltage loss (τVcable). The voltage loss is negligible when the secondary cables are short, but in ships, bridge piers, tanks, and the like, which are targets of the present invention, the voltage loss cannot be neglected because welding of a large welding length is performed by lifting a welder mounted with cables having a length of tens of meters while using a welding power source fixed on the ground.
For example, even when 37 V has been set as the power source output voltage Vpower that serves as a desirable welding condition, the arc voltage Varc varies depending on the lengths of the secondary cables. When the secondary cables are long and the voltage loss is large as illustrated in FIG. 12A, the arc voltage Varc, which is the difference between the power source output voltage Vpower and the voltage loss, becomes low. As a result, convection in a molten pool becomes weak and penetration becomes shallow. On the other hand, when the secondary cables are short and the voltage loss is small as illustrated in FIG. 12B, the arc voltage Varc, which is the difference between the power source output voltage Vpower and the voltage loss, becomes high. As a result, convection in a molten pool becomes strong and penetration becomes deep. Thus, it cannot be said that the penetration is controlled. Furthermore, although the penetration can be secured when the arc voltage is high, the mechanical properties of a weld metal may deteriorate when the arc voltage is excessive because the components of the weld metal become inappropriate due to significant oxidation reaction in the arc and inevitable mixing of the atmosphere in the arc.
In addition, in the case of downhand welding, even if the lengths of the secondary cables are not taken into consideration, failures in the penetration may be substantially prevented insofar as the arc is in contact with the surface of the groove, which may be adjusted by an operator during the welding. In the case of vertical welding, however, since the arc does not come into contact with the surface of the groove, it is difficult for the operator to determine whether or not appropriate penetration is being obtained. That is, even when current conditions or voltage conditions are inappropriate from the beginning, it is difficult for the operator to tell that. As a result, in the worst case, failures in the penetration and failures in the properties of the weld metal occur along the entirety of the welding length.
As described above, currently, only the control of the rising speed has been put into practice in a vertical welding apparatus, that is, only the shape of a weld portion is controlled, and the arc length and the voltage, which are two of other important welding conditions, are not controlled at all such that the arc length and the voltage become appropriate. Therefore, the stability of the penetration quality and the mechanical properties of the weld metal is substantially not controlled at all and fully dependent on the empirical intuition of the operator. For this reason, it has been desired to improve the apparatus to achieve further automation, elimination of the need for monitoring, and stabilization of quality.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 59-130689, changes in the arc length and the voltage, which is a quantitative value of the arc length, according to the rise of the surface of the molten pool are read and used to control a lifting motor, but this is the same as the above-described feedback control between changes in current and the lifting motor, and whether or not the absolute value of the arc length is appropriate is not controlled.
In addition, the techniques disclosed in Japanese Patent No. 3596723 and Japanese Unexamined Patent Application Publication No. 2004-167600, too, do not propose a method for controlling whether or not the absolute value of the arc length is appropriate.
An object of the present invention is to increase the possibility that the arc length is continuously maintained constant when vertical-up welding is performed by generating an arc in a groove between steel plates to be welded and forming a molten pool.